\section{Conceptual Design} % (fold)
\label{sec:Conceptual Design}

There are a number of differing ways in which such a stabilised platform could be built. First the operational requirements and environment were considered.

\subsection{Flight Profile} % (fold)
\label{sub:Flight Profile}

The proposed flight profile can be seen in Figure \ref{fig:flightprofile}. It is proposed that the vehicle will ascend under balloon to an altitude of approximately 30km, where a float will be initiated and sustained for several hours, over the course of a night, during which time the active stabilisation will be engaged and observations made. The vehicle will then terminate the flight at sunrise and begin its descent back to earth under parachute.

\begin{figure}[!htbp]
  \centering
  \includegraphics[width=0.8\textwidth]{concept/flightprofile.jpg}
  \caption{The proposed flight profile}
  \label{fig:flightprofile}
\end{figure}

It is proposed to use the largest commercially available latex weather balloon available, a Kaymont `3kg' balloon, where the mass refers to the mass of latex in the envelope \cite{kaymont}. The feasibility of this profile is calculated in Appendix \ref{sec:Flight Profile Calculations}.

\subsection{Design Considerations} % (fold)
\label{sub:Design Considerations}

A project like this presents a number of conflicting constraints in terms of cost, safety and reliability and time. As a result, it was not expected that a 'science-ready' prototype could be fully completed. Some of the constraints are outlined below. 
% subsection Design Considerations (end)

\subsubsection{Safety} % (fold)
\label{sub:Safety}

Safety has been of enormous importance throughout the project. As well as submitting a risk assessment to the Cambridge University Engineering Department, care has been taken to comply with Civil Aviation requirements specified in the Radiosonde Notice to Airmen (NOTAM) \cite{notam}. The system mass was defined to have an allowable upper limit of 5kg and the maximum descent rate undr parachute was set at 5ms$^{-1}$.

% subsection Safety (end)

\subsubsection{Environmental Conditions} % (fold)
\label{sub:Environmental Conditions}

The vehicle will be subject to severe environmental conditions. Over the course of a two hour ascent to operational altitude the vehicle could be exposed to temperatures as warm as 30$^\circ$C on the ground to -60$^\circ$ in the tropopause, where the air as at it's coolest \cite{nasaatm}. At 30km, the operation temperature at night will be -45$^\circ$C, which is below the limit of the `industrial' temperature range - the most robust temperature range in which commercially available components are offered \cite{indtemp}. Depending on the atmospheric conditions, moisture may form as the vehicle rapidly ascend through different pressure levels, which can then freeze onto delicate electronics and mechanical components.

%\begin{wrapfigure}{r}{0.4\textwidth}
%  \vspace{2.0cm}
%  \centering
%  \includegraphics[height=8cm]{concept/refframe.pdf}
%  \caption{Reference frame}
%  \label{fig:refframe}

%\end{wrapfigure}
%\subsubsection{Cost}
The envisaged budget for this vehicle was roughly \textsterling1000. This is a very large constraint on what is a fairly complex piece of mechatronics. A project undertaken by the author before this project used a three-axis accelerometer that cost \textsterling1300 to measure parachute deployment forces. A great deal of care was taken to get good performance from `cheap' components. 

% subsection Environmental Conditions (end)

\subsection{Concept selection} % (fold)
\label{sub:Concept selection}

\begin{wrapfigure}{r}{0.4\textwidth}
  \vspace{-4cm}
  \centering
  \includegraphics[height=8cm]{concept/refframe.pdf}
  \caption{Reference frame}
  \label{fig:refframe}

\end{wrapfigure}

Numerous methods of active stabilisation exist for tethered platforms under high altitude balloons. Passive stabilisation was dismissed based on the non-trivial disturbances induced by small amounts of wind sheer, couples induced by vortices being shed by the balloon, and `pendulum' motion. Previous experience suggests that even the most stable of flights have significant angular rates about yaw (for a definition of yaw, roll and pitch, see Figure \ref{fig:refframe}).

\subsubsection{Actuation} % (fold)
\label{ssub:Actuation}

Actuation about roll and pitch can be achieved with standard torque motors because the torque induced by the motors on the frame can be reacted against by gravity. However actuation in yaw is more challenging as there is nothing to react against. Three actuation strategies were considered and are compared in table

{\small\begin{table}[!ht]
  \label{tab:yawcom}
  \centering
  \begin{tabular}{p{4.5cm} p{4.5cm} p{4.5cm}}
	\toprule
	\multicolumn{1}{c}{\textbf{Reaction Wheel}} & \multicolumn{1}{c}{\textbf{Control Moment Gyro}} & \multicolumn{1}{c}{\textbf{Fans}} \\
	\midrule
	\includegraphics[scale=0.43]{concept/reactionwheel.pdf} & \includegraphics[scale=0.43]{concept/cmg.pdf} & \includegraphics[scale=0.43]{concept/blowers.pdf} \\
	{A reaction wheel is rotated about its axis by a torque motor. This generates an equal and opposite torque on its mounting point, rotating the gondola} & {A wheel is spun up to a fixed velocity. The assembly is rotated about an orthogonal axis $\theta_{gyro}$. A torque is generated by precession about an axis orthogonal to the first two, in proportion to $\dot\theta_{gyro}$.}  & {Fans on the end `outriggers' spin in opposing directions using the remaining trace of air to generate a couple about the yaw axis. The velocity of the fans effects the size of the couple.} \\
	\bottomrule

  \end{tabular}

  \caption{A comparison of yaw actuation systems}
\end{table}
}

Fans were rejected on the basis of their characteristics (such as total torque and the axial alignment of the couple) being a function of altitude, direction and velocity of swinging, which would be difficult to characterise. Control Moment Gyroscopes were rejected on the basis of the additional mechanical complexity (which would manifest itself in cost, mass and reliability) over reaction wheels. Reaction wheels were selected for their simplicity and ease of analysis.

% subsubsection Actuation (end)

\subsubsection{Electronics Overview} % (fold)
\label{ssub:Electronics Overview}

Figure \ref{fig:controloverview} shows the functional architecture of the control system. The microcontroller reads the values from the feedback sensors through a digital bus. A sensor fusion algorithm then estimates the state of the system, which is passed to the control law to calculate the torques required above the various axes to keep to a reference position. All of the main data is logged to the SD card for analysis during development, testing and post-flight.

\begin{figure}[ht]
  \centering
  \includegraphics[height=10cm]{concept/controloverview.pdf}
  \caption{A basic overview of the control system. Details from the final system omitted for clarity}
  \label{fig:controloverview}
\end{figure}

% subsubsection Electronics Overview (end)



% subsection Safety (end)

% subsection Concept selection (end)

% subsection Flight Profile (end)

% section Conceptual Design (end)